Wafer spin dryer and method of drying a wafer

ABSTRACT

In a spin dryer for semiconductor wafers, the wafer is held beneath a platen with its active side (i.e., the side containing the components or circuitry) facing upward. One or more nozzles spray rinse water on the top surface of the wafer and the wafer is rotated to remove the excess rinse water, thereby drying the wafer. A splash guard adjacent the edge of the wafer insures that the excess rinse water thrown off by the spinning wafer is deflected downward where it cannot again come into contact with the active side of the wafer. The platen is rotated dry at the same time, with no rinse water being splashed back onto the active side of the wafer. The spin dryer also includes a separate section which cleans and dries the end-effector of the robot which inserts the wafer into the spin dryer while the wafer is being dried.

FIELD OF THE INVENTION

This invention relates to a device for rinsing and drying flatsubstrates such as semiconductor wafers.

BACKGROUND OF THE INVENTION

In recent years it has become common in the semiconductor industry topolish the wafers after successive layers of conductive traces or otherstructures are formed on their surfaces. This produces very smoothsurfaces for repeated photolithographic processes and significantlyimproves the yield. The polishing process, typically chemical-mechanicalpolishing (CMP), leaves grit and other debris on the surface of thewafer which must be removed before fabrication process can continue.This has led to the development of wafer cleaning devices, such as thedevice described in co-pending application Ser. No. 08/683,654, which isincorporated herein by reference in its entirety. Following cleaning,the wafers must be thoroughly dried before they can be returned to thefabrication line.

Wafer spin dryers use a combination of centrifugal force and air flow toremove all moisture from the surface of the wafer. Two known types ofspin dryers are illustrated in FIGS. 1 and 2. In the version shown inFIG. 1, a wafer 10 is held by fingers 12 above a platen 13. Platen 13 isrotated on a shaft 14 by a spin motor 15. Rinse water is applied fromabove by one or more nozzles 16 to the "good" (active) side of wafer 10.A problem with this type of dryer is that the drying chamber 17 is opento the atmosphere, which while normally quite clean still containsparticulate matter. Particles which fall onto the wafer during andimmediately following the drying operation will remain when the nextprocess step begins.

In the prior art dryer shown in FIG. 2, the drying chamber 20 is notopen to the atmosphere from above, and the wafer 10 is held by fingers21 below the platen 22. The platen is spun by a motor 23 which ismounted above the platen. The good side of the wafer faces downward, andone or more nozzles 24 apply rinse water 10 from below. A problem withthis type of dryer is that water droplets which are thrown from thespinning wafer can strike the surface of the drying chamber and splashagainst the good side of the wafer.

Thus there is a need for a wafer dryer which avoids the above problemsand reliably yields a clean, dry wafer suitable for further processing.

SUMMARY OF THE INVENTION

The wafer spin dryer of this invention includes a platen and a pluralityof holding members or fingers which extend downward from the platen. Thewafer is held with its good or active side facing upward. One or morenozzles are positioned so as to direct a rinse liquid (typically water)against the good side of the wafer. The rinsing liquid is applied to thegood side of the wafer, and the wafer is rotated to create a centrifugalforce which removes the liquid from the good side of the wafer. Asurface laterally adjacent the edges of the spinning wafer is contouredand angled such that the liquid which flies from the wafer is directeddownward to a portion of the drying chamber below the wafer. As aresult, the used rinse liquid cannot come into contact with the goodside of the wafer.

The wafer is preferably placed in the spin dryer by a robot. In thepreferred embodiment, the wafer is held in the spin dryer by threefingers which extend downward from the platen and which are spaced atequal (120°) angles around the axis of rotation. The fingers containnotches or other concave surfaces which grip the edge of the wafer. Oneof the fingers is movable to allow the robot to place the wafer in aposition where it can be held by the three fingers. The wafer ispositioned slightly eccentric to the axis of rotation such that it isforced against the two fixed fingers as it is rotated. The rotating mass(wafer and platen) is balanced as a whole so that undue vibrations donot occur as the wafer is rotated.

In many situations, the same robot arm places the wafer into thecleaner, transfers the wafer from the cleaner to the dryer, andtransfers the wafer from the dryer to the finished wafer cassette. Theend-effector of the robot arm can thus become contaminated with grit andchemicals. As another aspect of this invention, the wafer dryer containsa separate chamber which is used to clean and dry the end-effector whilethe wafer is being dried. Thus, when the wafer is withdrawn from thespin dryer, it does not become re-contaminated with grit and/orchemicals from the end-effector.

In the preferred embodiment the end-effector cleaning chamber containsone or more nozzles which spray a rinse liquid onto the end-effector andone or more nozzles which direct a flow of nitrogen against theend-effector and sweep the rinse liquid from the end-effector as it isbeing withdrawn from the end-effector cleaning chamber.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a prior art wafer spin dryer in which the wafer ispositioned above the platen with its good side facing up.

FIG. 2 illustrates a prior art wafer spin dryer in which the wafer ispositioned below the platen with its good side facing down.

FIG. 3 is a general view of a wafer cleaning/drying system, includingthe wafer spin dryer of this invention and a robot for transferring thewafers between units.

FIG. 4 is a broken away perspective view of a wafer spin dryer of thisinvention.

FIGS. 5A and 5B are broken away side and end views, respectively, of thewafer spin dryer in an open position allowing the introduction of awafer.

FIGS. 6A and 6B are broken away side and end views, respectively, of thewafer spin dryer in a closed position.

FIG. 7 is a cross-sectional view of the platen and associatedmechanisms.

FIG. 8 is a broken away view of the mechanism for pivoting one of thefingers used to grip the wafer.

FIG. 9 is an exploded view of the mechanism for lifting and lowering thehood unit of the spin dryer.

FIG. 10 is a top view of the end-effector cleaning section.

FIG. 11 is an exploded view of the end-effector cleaning section.

DESCRIPTION OF THE INVENTION

A wafer cleaning/drying system 30 is shown in the general perspectiveview of FIG. 3. System 30 includes a wafer spin dryer 32 in accordancewith this invention. Also shown are a buffer unit 34, a wafer cleaningunit 36, a robot 38, and a finished wafer cassette 39. Robot 38 gripsthe wafers with a vacuum actuated end-effector 38A of a kind widely usedin the semiconductor processing industry. In normal operation, thewafers are placed into the buffer unit from a CMP unit. Robot 38 takesthe wafers from buffer unit 34 and inserts them into the cleaning unit36, where they are scrubbed and rinsed. Wafer cleaning unit 36 ispreferably of the kind described in the above-referenced applicationSer. No. 08/683,654. After the wafers have been cleaned, robot 38removes them from the cleaning unit 36 and inserts them into the waferspin dryer 32. After further rinsing and drying, the wafers are placedinto the finished wafer cassette 39.

The operation of the various units in system 30 is controlled by aprogrammable logic controller (PLC) (not shown) which may, for example,be a Model 2600, manufactured by Control Technology Corp. of Hopkinton,Mass. The PLC is controlled from a control panel 37.

FIG. 4 shows a broken away perspective view of wafer spin dryer 32 takenfrom the other side as compared with the view of FIG. 3. Wafer spindryer 32 includes a hood unit 40, which is movable vertically to allowwafers to be inserted into the spin dryer, and a base unit 41. Hood unit40 includes a slot 42 at the entrance of a section (described below)which is used to clean and dry the end-effector 38A of robot 38 while awafer is being dried in spin dryer 32.

FIGS. 5A and 5B are broken away side and end views, respectively, ofwafer spin dryer 32 in an open position, with hood unit 40 in a raisedposition, allowing a wafer to be inserted into wafer spin dryer 32. Hoodunit 40 preferably includes an external housing 50 of plastic, a topplate 51 also formed of plastic, and a horizontal mounting plate 52formed of sheet metal. The housing of hood unit 40 is assembled in aconventional manner with plastic welding.

Base unit 41 includes a base plate 53 and a cylindrical side housing 54for a drying chamber 55. A bracket 56 is attached to the inside surfaceof side housing 54. Extending upward from bracket 56 is a conical splashguard 57, and extending downward from bracket 56 is a conical floor 58of drying chamber 55. Floor 58 slopes down to a drain 59 which islocated at the lowest point in drying chamber 55 to remove any rinsewater which accumulates in chamber 55 during the drying process. Drain59 extends through a manifold block 60 which is mounted on base plate53. Manifold block 60 is used to supply water to a set of nozzles 61which are used to direct rinse water upward at the back side of thewafer. A lower edge of floor 58 is welded to manifold block 60.

A second set of nozzles 62 are mounted in a pair of manifold blocks 63Aand 63B which are attached to side housing 54 as shown in FIG. 5A.Nozzles 62 are positioned such that they direct a stream or spray ofrinse water against the good side of a wafer when spin dryer 32 is inthe closed position, with hood unit 40 lowered (see FIGS. 6A and 6B). Asis evident from FIG. 5B, each of manifold blocks 63A and 63B suppliestwo nozzles 62, and manifold blocks 63A and 63B are positioned onopposite sides of the centerline of drying chamber 55. Nozzles 62 areoriented so as to direct a stream or spray of rinse water downward at anangle against the good side of a wafer.

Manifold blocks 63A and 63B are preferably made of plastic and manifoldblock 60 is preferably made of stainless steel. Manifold blocks 60, 63Aand 63B have channels and cavities formed therein in a customary mannerto supply the rinse water to the respective nozzles. Rinse water issupplied to manifold blocks 60, 63A and 63B via fittings and tubes whichare generally not shown in FIGS. 5A, 5B, 6A and 6B. The flow of rinsewater into the manifold blocks and nozzles is controlled by valves whichare remote from the manifold blocks and which are in turn controlled bythe PLC in wafer spin dryer 32. The detailed structure of these elementshas been omitted from the drawings for the sake of clarity but will bereadily understood by persons skilled in the art.

Referring again to FIGS. 5A and 5B, a spin motor 64 is mounted on theupper side of horizontal mounting plate 52. A platen 65 is positionedbelow spin motor 64 and is driven by a drive shaft 66. The details ofthis structure are shown in the cross-sectional view of FIG. 7. Motor 64is mounted on a mounting block 67, which is attached to the bottom sideof horizontal mounting plate 52 with screws. Drive shaft 66 is connectedto a hub 68 by means of a collet 69. The top side of platen 65 containsa recess into which hub 68 fits, and platen 65 is attached with screwsto hub 68. Platen 65 is a circular plate of polypropylene which in thepreferred embodiment is about 0.5 inches thick. To remove any airborneparticles which might be generated by the bearings of motor 64, a vacuumis applied through a tube 64A and a fitting 64B to a cavity 64C which isformed in mounting block 67 (see FIG. 5A). Cavity 64C communicates withthe space surrounding hub 68, and thus any particles generated by themotor bearings are drawn through tube 64A.

Referring again to FIG. 5B, fingers 69A, 69B and 69C extend downwardfrom platen 65 for gripping wafer 10. Finger 69A is located near theentrance 70 through which wafers are inserted into spin dryer 32, andfingers 69B and 69C are located away from the entrance. As is evidentfrom FIG. 5A, finger 69A can be pivoted so as to permit wafer 10 to bepositioned between fingers 69A, 69B and 69C. FIG. 7 also shows notches69D that are formed in fingers 69A, 69B and 69C. When finger 69A is inits normal, unpivoted position the edges of wafer 10 fit within notches69D, and wafer 10 is held in a fixed position below platen 65. Asdescribed further below, the good or active side of wafer 10 (i.e., theside on which the electronic circuitry and components are formed) facesupward in the direction of platen 65 during the drying process. Fingers69A, 69B and 69C are positioned at equal angular intervals about thecentral axis of wafer 10 when wafer 10 is held in position for drying.

FIGS. 7 and 8 illustrate the mechanism used to pivot finger 69A. Finger69A is mounted in a slot 71 in platen 65, which permits finger 69A topivot about a horizontal axis on a pin 72 between a vertical position,where a wafer is held in place, and a tilted position, which allows awafer to be inserted between fingers 69A, 69B and 69C.

A spring plunger 73 is mounted in a cavity in platen 65 adjacent finger69A. Spring plunger 73 contains an actuator 74 which presses laterallyagainst finger 69A at a location above pin 72 and thereby urges finger69A into its vertical position.

An annular cavity 75 is formed in mounting block 67, and a pneumaticcylinder 76 is mounted in the cavity. Pneumatic cylinder 76 has aclub-shaped actuator 77 which extends radially outward. When pneumaticcylinder 76 is in its normal position, shown in FIG. 7, there is a smallclearance between actuator 77 and a top end 78 of finger 69A. Whenpneumatic cylinder 76 is actuated, actuator 77 is pulled to the right(in FIG. 7) and causes finger 69A to pivot about shaft 72 to its tiltedposition. Spin motor 64 is preferably a servo motor and thus can becontrolled to stop at a selected angular position with finger 69Alocated radially inward from club-shaped actuator 77.

The details of the mechanism used to lift and lower hood unit 40 isshown in FIG. 9, although many alternative techniques for accomplishingthis will be apparent to those skilled in the art. A rear support plate90 and side support plates 91 and 92 are mounted on base plate 53. Apneumatic lifting mechanism 93 is mounted inside the support plates 90,91 and 92, with a top member 94 and a bottom member 95 being bolted torear support plate 90. Air pressure tubes are connected to pneumaticlifting mechanism 93 and are controlled to cause a lifting member 96 torise and fall. Lifting member 96 is bolted to a vertical interior wallwithin hood unit 40, thereby enabling hood unit 40 to rise and fall withlifting member 96. Lifting mechanism 93 is advantageously the rodlesscylinder manufactured by Tol-o-matic of Minneapolis, Minn.

The operation of spin dryer 32 will now be described. Initially, liftingmechanism 93 causes hood unit 40 to move to its raised position, therebyopening entrance 70 to the interior of drying chamber 55. Robot 38removes wafer 10 from cleaning unit 36, using its vacuum actuatedend-effector 38A, and inserts wafer 10 into drying chamber 55 with thegood or active side of wafer 10 facing upward. Pneumatic cylinder 76 isactuated. Since platen 65 is in its "home" index position, withclub-shaped actuator 77 adjacent upper end 78 of finger 69A, this causesfinger 69A to pivot to its tilted position. Robot 38 lifts wafer 10 to aposition between fingers 69A, 69B and 69C, at the level of notches 69D,and then adjusts the lateral position of wafer 10 until the edge ofwafer 10 comes into contact with the notches 69D of fingers 69B and 69C.Pneumatic cylinder 76 is then actuated so as to allow spring plunger 73to force finger 69A to its vertical position, firmly clamping wafer 10in notches 69D of fingers 69A, 69B and 69C. The sides of notches 69D aresloped at an angle (e.g., 45°) which allows for a small error in thevertical positioning of wafer 10 by robot 38 while insuring that wafer10 is properly seated in notches 69D.

From FIG. 7 it will be noted that the center X₁ of wafer 10 is slightlydisplaced in the direction of fixed fingers 69B and 69C from the axis ofrotation X₂ of platen 65 and wafer 10. For an eight-inch wafer, thisdisplacement D is approximately 1/8". This insures that as wafer 10rotates it presses against the fixed fingers 69B and 69C rather than themovable finger 69A. If wafer 10 were to press against movable finger69A, it might overcome the force of spring plunger 73 and cause finger69 to pivot, releasing wafer 10 from the grip of fingers 69A, 69B and69C. Platen 65 is appropriately counter-balanced to compensate for theoffset position of wafer 10 and avoid vibrations from occuring whenwafer 10 is being rotated.

With wafer 10 gripped by fingers 69A, 69B and 69C, the end-effector 38Ais withdrawn through entrance 70. After the wafer spin dryer 32 has beenclosed to begin the drying process, end-effector 38A is inserted throughslot 42 into the end-effector cleaning section (described below).

Lifting mechanism 93 is pneumatically actuated so as to lower hood unit40, thereby closing off entrance 70. As shown in FIGS. 6A and 6B, aresilient ring or bead 88 is fixed to the top edge of side housing 54.When hood unit 40 is in its lowered position, bead 88 presses againstthe lower surface of mounting plate 52 to seal off drying chamber 55from the outside environment.

Spin motor 64 is turned on to rotate platen 65 and wafer 10 at arelatively slow speed (e.g., 100 rpm) and rinse water (preferablydeionized water) is sprayed through nozzles 62 onto the top (active)side of wafer 10. At the same time, rinse water is sprayed throughnozzles 61 against the back side of wafer 10. This continues forapproximately 10-15 seconds, at which time nozzles 61 and 62 are turnedoff and the rotational velocity of spin motor is increased to a muchhigher level (e.g., 5000 rpm). At this speed, centrifugal force causesthe rinse water on the surfaces of wafer 10 to flow toward the edge ofthe wafer where it is thrown radially outward.

As shown in FIGS. 6A and 6B, when hood unit 40 is in its loweredposition, wafer 10 is positioned well below the top edge of splash guard57. Therefore, as rinse water is thrown from the edges of wafer 10, itstrikes splash guard 10 and is deflected downward toward the portion ofdrying chamber 55 below wafer 10. None of this rinse water can splashback to the top surface of wafer 10, and thus the top (active) surfaceof wafer 10 is thoroughly and effectively dried. The rinse water runsdown the sloping floor 58 and through drain 59.

The drying process normally lasts for about 45 seconds. Spin motor isthen turned off, and lifting mechanism 93 is pneumatically actuated soas to raise hood unit 40, thereby opening entrance 70. Robot 38 insertsend-effector 38A (which by now has been cleaned) into drying chamber 55,and raises end-effector 38A until it is in contact with the lowersurface of wafer 10. Pneumatic cylinder 76 is actuated to tilt finger69A, releasing wafer 10 from fingers 69A, 69B and 69C, and robot 38moves wafer 10 a short horizontal distance in the direction of finger69A, insuring that the edge of wafer 10 is clear of notches 69D offingers 69B and 69C. Robot 38 then lowers wafer 10, withdraws wafer 10through entrance 70, and places wafer 10 in finished wafer cassette 39.This completes the wafer drying process.

As described above, hood unit 40 also contains a end-effector cleaningsection which is accessible through slot 42. End-effector cleaningsection 100 is shown in FIGS. 5A and 6A. FIG. 10 is a top view ofcleaning section 100, including end-effector 38A, and FIG. 11 is anexploded view of cleaning section 100. Included are a top plate 102 anda bottom plate 104, preferably made of stainless steel, and spacers 106and 108, preferably made of plastic. Plates 102 and 104 and spacers 106and 108 are fastened together with screws, creating an internal cavitywhich is shaped to fit end-effector 38A.

A series of holes 110A and 110B are drilled in top plate 102, and acorresponding series of holes 112A and 112B are drilled in bottom plate104. Holes 110A, 110B, 112A and 112B are used to supply jets of heatednitrogen into the internal cavity of cleaning section 100, and they arepreferably drilled at an angle of about 45° so that the flow of nitrogenis directed away from slot 42 and into the internal cavity of cleaningsection 100. The nitrogen is directed into holes 110A and 110B through atop manifold 114 and into holes 112A and 112B though a bottom manifold116. Manifolds 114 and 116 are clamped to top plate 102 and bottom plate104, respectively, with screws (not shown). Using bottom manifold 116 asan example, a cavity 116A registers with holes 112A, and a cavity 116Bregisters with holes 112B. Nitrogen flows through a tube (not shown) anda fitting 115 (FIG. 6A) into cavities 116A and 116B and is ejectedthrough holes 112A and 112B into the internal cavity of cleaning section100. Top manifold 114 contains similar cavities with register with holes110A and 110B and permit nitrogen to be ejected into the internal cavityof cleaning section 100 through holes 110A and 110B. Cavities 116A and116B are connected with the corresponding cavities in top manifold 114by means of a series of aligned holes that are formed in plates 102 and104 and spacer 108. The nitrogen is heated to a temperature of 140° F.by a heater (not shown) which is located in wafer cleaning/drying system30.

Top plate 102 also contains a pair of holes 118 which are used to spraywater into the internal cavity of cleaning section 10, and bottom platecontains a similar pair of holes 120. Holes 118 are angled similarly toholes 110A and 110B, and holes 120 are angled similarly to holes 112Aand 112B. Bottom manifold 116 contains a cavity 116C which registerswith holes 120 and which is connected through holes in plates 102 and104 and spacer 108 to a similar cavity in top manifold 114 whichregisters with holes 118. Water flows through a tube (not shown) and afitting 117 (FIG. 6A) into cavity 116C.

When robot 38 has finished inserting wafer 10 into spin dryer 32, itinserts end-effector 38A through slot 42 into the internal cavity ofcleaning section 100. Rinse water is sprayed through holes 118 and 120against the top and bottom surfaces of end-effector 38A, removing gritand/or chemicals which have accumulated on the end-effector in thecourse of transferring the wafer from buffer unit 34 to cleaning unit 36and to spin dryer 32. The rinse water exits cleaning section 100 througha drain 122. As the end-effector 38A is withdrawn from cleaning section100, heated nitrogen is supplied through holes 110A, 110B, 112A and112B. The jets of heated nitrogen "wipe" the rinse water from thesurfaces of end-effector 38A and dry the end-effector before it iscalled upon to remove wafer 10 from spin dryer 32.

It is to be understood that the foregoing description is illustrativeand not limiting. For example, while the embodiment described above isfor drying a semiconductor wafer, the principles of this invention maybe applied to drying other types of flat substrates. Many alternativeembodiments in accordance with the broad principles of this inventionwill be apparent to persons skilled in the art.

I claim:
 1. A spin dryer comprising:a rotational member positioned in a drying chamber of said spin dryer, said rotational member being attached to a rotatable shaft; a spin motor for rotating said shaft; a plurality of gripping members extending downward from said rotational member, said gripping members being adapted so as to hold a flat substrate below said rotational member; and at least one nozzle mounted in said spin dryer, said nozzle being positioned so as to direct a spray of liquid onto a top surface of a substrate held by said gripping members.
 2. The spin dryer of claim 1 further comprising a semiconductor wafer, said semiconductor wafer being held by said gripping members such that an active side of said semiconductor wafer faces upward.
 3. The spin dryer of claim 1 further comprising at least one nozzle positioned so as to direct a spray of liquid onto a bottom surface of a substrate held by said gripping members.
 4. The spin dryer of claim 1 wherein said plurality of gripping members comprises three fingers, each of said fingers having a surface contoured for gripping an edge of said substrate.
 5. The spin dryer of claim 4 wherein one of said fingers is movable with respect to said rotational member.
 6. The spin dryer of claim 5 further comprising a mechanism for moving said one finger from a first position, wherein a substrate can be inserted between said fingers, and a second position, wherein a substrate is held by said three fingers.
 7. The spin dryer of claim 6 further wherein said mechanism comprises a spring mechanism, said spring mechanism urging said movable finger toward said second position.
 8. The spin dryer of claim 7 further comprising a mechanism for overcoming a force of said spring mechanism so as to move said one finger to said first position.
 9. The spin dryer of claim 8 wherein said mechanism for overcoming a force of said spring mechanism comprises a pneumatic cylinder.
 10. The spin dryer of claim 5 wherein said movable finger is pivotable about a horizontal axis.
 11. The spin dryer of claim 5 wherein two of said fingers are fixed with respect to said rotational member.
 12. The spin dryer of claim 1 further comprising a splash guard located near an edge of said substrate, said splash guard comprising a surface for directing droplets thrown laterally from an edge of said substrate to a region below said substrate.
 13. The spin dryer of claim 1 further comprising a section for cleaning a robot end-effector.
 14. The spin dryer of claim 13 wherein said section comprises a cavity for receiving said end-effector and at least one nozzle for directing a flow of liquid against said end-effector.
 15. The spin dryer of claim 14 wherein said section further comprises at least one nozzle for directing a flow of gas against said end-effector.
 16. A combination comprising the spin dryer of claim 1 and a semiconductor wafer held by said gripping members in a position below said rotational member, an active side of said wafer facing said rotational member.
 17. The spin dryer of claim 1 wherein said rotational member comprises a platen.
 18. The spin dryer of claim 1 wherein said drying chamber is sealed from an outside environment.
 19. A spin dryer comprising:a rotational member positioned in a drying chamber of said spin dryer, said rotational member being attached to a rotatable shaft; a spin motor for rotating said shaft; and a plurality of gripping members extending from said rotational member, said gripping members being adapted so as to hold a circular substrate, at least one of said gripping members being movable with respect to said rotational member and several of said gripping members being fixed with respect to said rotational member; wherein said gripping members are positioned with respect to said shaft such that a center of said circular substrate is displaced by a predetermined distance from an axis of rotation of said rotational member when said circular substrate is engaged by said gripping members.
 20. The spin dryer of claim 19 wherein said center of said circular substrate is displaced from said axis of rotation in a direction generally away from said at least one movable gripping member.
 21. The spin dryer of claim 20 comprising three of said gripping members, one of said gripping members being movable with respect to said rotational member and two of said gripping members being fixed with respect to said rotational member. 